Transmitted laser beams, while ideally perfectly collimated, exhibit a certain amount of divergence. The effect of this divergence is to weaken the density of a laser beam which is transmitted over significant distances. In order to reduce this divergence, beam expanders are used to initially expand the laser beam. This results in the laser beam being expanded in diameter by a modification factor: ##EQU1## By transmitting the laser beam through a beam expander, the angle of divergence is decreased by the magnification factor of a beam expander. Hence beam expanders are used where it is desired to concentrate laser energy transmitted across distances. In those cases, the divergence angle of the laser beam is a significant factor in determining the energy intensity of a laser beam.
The beam expander itself may cause the beam to diverge or converge. In certain applications the beam expander is set to initially diverge the beam. Then, the beam divergence is changed until the best collimation is achieved. The beam may even be caused to converge. For that reason, in order to reduce beam divergence, it is desired to fine tune the optics of a beam expander.
Adjustments are made in prior art beam expanders (FIG. 1) by effecting longitudinal translation of one of the lens groups 13 in the beam expander 11 along its optical axis. In an alignment telescope, for example, the translating lens 13 moves in a lapped cylinder 21 to minimize side-to-side motion that affects beam directivity. A small clearance space between the outside diameter of a lens housing 20 and the inside diameter of the lapped cylinder 21 must be present to permit assembly and to allow for thermal expansion. However, the lens 13 is not fully constrained and tilt and decentralization can occur when the lens 13 is longitudinally shifted. This perturbs the directivity of the beam axis. It is desired not to perturb the directivity of the beam axis because that would remove the laser energy from its intended target.
Accordingly, it is desired to provide a laser beam which has a reduced divergence angle for maximum energy density at a target. It is desired to provide a beam expander to reduce the angle of divergence and to provide for adjustment of the beam expander in a manner which would avoid or greatly reduce tilt and decentralization of the beam.